Due to the impressive development of fiber laser technology in recent years [1], optical fibers have earned a solid reputation as a highly power scalable laser concept. This unparalleled progress, that has seen laser systems evolve from low power setups to multi-kW industrial systems in about a decade, has much to do with the extremely high-power handling capability offered by the geometry of the fiber. The very high surface to active volume ratio allows for an efficient heat removal and, therefore, for high-power operation. However, even though the geometry of the fiber relaxes the demands on thermal management, it generates other problems. Thus, the tight confinement of the light in the core of the fiber gives rise to high intensities that interact with the fiber material over long lengths, which increases the impact of non-linear effects. Hence, active fibers for high-power operation (especially in pulsed operation) have to be specifically designed to alleviate the adverse consequences derived from the non-linearity of the material. The most effective way of mitigating non-linear effects in active fibers is to enlarge the core. This results in a two-fold advantage: on the one hand it reduces the intensity of the light propagating in the fiber core and, on the other hand, in double-clad fibers if the pump cladding diameter is not changed, it increases the pump absorption, which allows for shorter devices, thus further mitigating the impact of non-linear effects. Unfortunately, realizing fibers with large cores that still support single-mode operation is far from trivial, especially for high-power operation. In fact, even though the most advanced fiber designs have some in-built mechanism of mode discrimination [2-3], fibers with mode-field diameters larger than 50 μm typically support the propagation of a few modes. Consequently, in high-power fiber laser systems today the combination of high thermal loads with few-mode operation is to be found for the first time [4]. This can potentially give rise to new phenomena such as the recently observed onset of mode instabilities at high average powers [5].
The phenomenon of mode instabilities refers to the output beam of a fiber laser system becoming suddenly unstable once that a certain output power threshold has been reached. Thus, it can be observed that with only a small increase of the output power, the once Gaussian-like output beam of the fiber starts to fluctuate. In this regime the intensity profile at the output of the fiber shows a constantly changing beam formed by the coherent superposition of the fundamental mode and one or more higher-order modes [6]. Recent measurements with a high-speed camera have confirmed that there is actually energy transfer between the fundamental mode and the higher-order mode [7]. Furthermore, Fourier analysis of the beam fluctuations has revealed that, near the threshold, these are not random but follow quasi-periodic patterns with well-defined frequencies [8]. However, when increasing the power further, the beam fluctuations seem to become chaotic.
Shortly after the first reports of this effect came out, the first hypothesis on the origin of the effect was published [9]. In this explanation the interference pattern that appears along a fiber due to the beating of two transverse modes gives rise to a long period index grating via the thermo-optic effect or the resonantly enhanced non-linearity of active fibers [10]. Even though this theory could not explain the dynamic behavior of mode instabilities, it provided an explanation for the mechanism responsible for the energy transfer between two orthogonal transverse modes.
Over the last years several mitigation strategies for mode instabilities have been proposed, demonstrated and/or patented. Some of these rely on the delocalization of higher-order modes [11], on exploiting fiber designs with confined-doping [12], on using gain saturation [13], on reducing the quantum defect in the laser system [14] or on reducing the laser-active-ion concentration in the fiber core [15], just to name a few. Most of the mitigation strategies proposed up to now for mode instabilities are passive ones, i.e. they do not require any external feedback loop to operate. There is, however, an already experimentally demonstrated way of actively stabilizing a fluctuating beam above the mode instability threshold using an acousto-optic deflector [16]. The idea hereby was to dynamically change the coupling conditions in the fiber to wash-out the thermally-induced index grating responsible for mode instabilities. Using this technique a beam could be stabilized at powers that were 4 times higher than the original mode instability threshold.